Department of Electrical and Computer Engineering, University of British Columbia, Vancouver, British Columbia V6T1Z4, Canada.

Abstract

The development of bendable, stretchable, and transparent touch sensors is an emerging technological goal in a variety of fields, including electronic skin, wearables, and flexible handheld devices. Although transparent tactile sensors based on metal mesh, carbon nanotubes, and silver nanowires demonstrate operation in bent configurations, we present a technology that extends the operation modes to the sensing of finger proximity including light touch during active bending and even stretching. This is accomplished using stretchable and ionically conductive hydrogel electrodes, which project electric field above the sensor to couple with and sense a finger. The polyacrylamide electrodes are embedded in silicone. These two widely available, low-cost, transparent materials are combined in a three-step manufacturing technique that is amenable to large-area fabrication. The approach is demonstrated using a proof-of-concept 4 × 4 cross-grid sensor array with a 5-mm pitch. The approach of a finger hovering a few centimeters above the array is readily detectable. Light touch produces a localized decrease in capacitance of 15%. The movement of a finger can be followed across the array, and the location of multiple fingers can be detected. Touch is detectable during bending and stretch, an important feature of any wearable device. The capacitive sensor design can be made more or less sensitive to bending by shifting it relative to the neutral axis. Ultimately, the approach is adaptable to the detection of proximity, touch, pressure, and even the conformation of the sensor surface.

(A) Mutual capacitive coupling is simulated between two planar electrodes (shown in the inset without a finger). The coupling between electrodes is reduced by the presence of a finger, which acts as an electrode itself. (B) Finger approaching a pair of electrodes that are in the form of a loop and disc. The finger reduces the coupling between the electrodes (CM) by coupling itself with the projected field (CF is increased). (C) Two-dimensional array of loop-disc electrode pattern, with the loops on top. (D) Sensor array sitting above a forearm and a printed number pad. (E) Sensor array on an LCD with a video playing demonstrating transparency. Two edges of the sensor are indicated by the dashed lines. A third edge is just visible, extending perpendicular to the lower line. (F) Sensor array wrapped around a finger demonstrating conformity.

(A) Curing PDMS in a mold. (B) Plasma-bonding three layers forming perpendicular channels on top and bottom of the dielectric. (C) Injecting the monomer mixture inside the channels and polymerizing them to form the ionically conducting electrodes. (D) Map showing the localized change in capacitance due to a touch by a finger. (E) Change in capacitance due to a hovering finger at various distances from the top of the sensor. The change in capacitance upon approach of the finger is negative, as indicated.

(A) Plot showing the recorded displacement (7% strain) and capacitance of a single taxel in the sensor while being stretched by a sinusoidally varying displacement using a dynamic mechanical analyzer; the sharp drops in capacitance coincide with touch, while the sinusoidally varying changes are the result of the stretching. (B) Sensor in steady state. (C) Stable capacitance map for steady state. (D) Sensor folded in an anticlockwise direction. (E) Resulting small positive change in capacitance of the taxels along the axis of bend where capacitance increases with positive y axis. (F) Sensor being touched while being bent. (G) Negative change in capacitance map showing the taxel touched having a change in capacitance of 10%, reduced slightly by the change due to the bend where the capacitance decreases with positive y axis. (H) Sensor being bent in the clockwise direction. (I) Positive change in capacitance map showing the axis of bend and a similar response to that in (E).

(A) Swipe functionality of the sensor showing the negative change in capacitance following a movement across the row from left to right. (B) Detection of one (top), two (middle), and three (bottom) fingers simultaneously, demonstrating multitouch capability. (C) Diagram of the design and results from an augmented bend sensor (compared with the regular sensor with the neutral axis aligned with the dielectric), with the neutral axis aligned with the top electrode layer, which enhances the detection of bend (bottom left) but still enables the simultaneous detection of touch (bottom right).